I have a relatively large house (5000 square feet) with a natural gas powered tankless hot water heater paired with a recirc pump. We've been in the house for three years and have recently started to have some trouble with our hot water. This has led me to want to better understand the system as a first step towards diagnosing the problem we're having.

The system:
Takagi TK3 flash hot water heater
Large'ish copper/brass recirc pump plumbed in to what appears to be a hot water loop
Invensys SR301 3 zone relay which appears to be controlling the pump

The problem:
On occasions, when you turn on the hot water you get hot water for a while, then it goes cold for a while, then hot again and it will stay hot. This is particularly problematic in the shower scenario where you turn on the water, notice that it's hot, get half way through your shower, then get ice cold water for 3 minutes before the hot water returns.

My Questions:
1. What role does the 3 zone relay play when I don't have multiple zones? I assume the hot water loop is just a single zone and that there'd be a simple timer on the recirc pump rather than this big complex looking unit.

2. I can imagine there being two different failures that might cause the behavior we're seeing. A) The recirc pump not running when it should such that there's a bunch of cold water in the lines. If that were the case though I would think we'd get cold water for a while before getting hot but once we got hot water it would stay hot. B) The hot water heater isn't kicking on quickly anymore for some reason such that we get the initial blast of hot water that was stored in the lines, then cold water as the heater is trying to cycle, and then finally hot water again once it successfully starts.

I'm guessing it's B?

3. If it is A, I assume I need to tweak the programming of the relay? I think it's programmable. If it's B, I assume I need to figure out why the hot water heater isn't starting quickly?

I've read that recirc pumps and tankless hot water heaters often don't play well together and wouldn't be surprised if the builder's subcontractor did something dumb with my set-up. There have been a bunch of other things that we've discovered that are suboptimal and stemmed from going with the cheapest sub. Reading the TK3 manual seems to indicate that this particular unit will work with recirc pumps if set-up correctly.

Sorry for the length of my post. I wanted to make sure I gave you guys all the information that might help.

I don't know the specifics of that tankless, but some of them just don't like recirculation. In fact, on some of them, it voids the warranty. If they do allow recirculation, it takes some fairly specific plumbing and maybe a buffer tank.

Some tankless (and boiler) systems have a safety that will shut them down for a moment if they've been running for some period of time. This would be described in the user's manual. Also, over time, the tankless will become 'limed up' with mineral deposits, and is no longer as efficient on transferring the heat to the water through that caked on buildup. This could affect the sensors as well. They need periodic acid flushes to remove that buildup and maintain proper and efficient operation.

If that dip in temp isn't caused by a safety cutout, then it may be one of two other things: you're running a cooler shower that isn't tripping the flow sensor because you're not using all that much hot water, or the flow sensor is sticky and not tripping properly.

They may have used a 3-zone controller because it was easier to find. The price difference isn't much, and wiring it up is essentially the same, too. And, it gives you a couple of spare relays, should the one you're using crap out.

Yeah, I saw lots of information about tankless hot water heaters not being supported with recirc pumps for various reasons. The manual on this TK3 said it has a built in controller for recirc pumps and supports them. Of course that doesn't mean mine is set-up correctly for one.

I've been wondering if there's some sort of maintenance I should do. It sounds like I should look into this mineral cleaning process. I lived in West Texas for a while and that water was AWEFUL for leaving deposits. I live in Washington state now and the water seems to be way lower in mineral content but I'll look into the cleaning thing. Thanks for that suggestion.

I run a pretty hot shower and we used to not have the problem so I don't think cool shower is it.

Good point on the 3-zone controller being easier to get as a possible solution for why it's there vs. a simple timer.

The controller is a pretty simple device, if it's what I think it is...it lets a 24vac thermostat/aquastat turn the 120vac power on/off to the pump...it could have a timer wired in, but in itself, all it is is a voltage changer/switch. It is normally used in a heating system. On the 3-zones, when any one zone calls for heat (or in your case recirculation), it also would turn on the boiler (and maybe your tankless system uses that input as well).

Interesting. So it's really just a switch. That would explain why the manual doesn't mention anything about setting a temperature or time. I don't *think* the hot water heater has a timer built in but it does have a temperature setting I think. I'll re-read the manual.

I wonder if I wouldn't be better off getting rid of the controller and just using a simple timer to control whether the pump is on or not. Basically I'd have it on for a few hours in the morning when we're likely to shower and in the evening for the same purpose.

There are two possibilities;
1. The return line does not have a check valve, which could cause your symptoms, and
2. You have a tempering valve so it #1 is not the problem then the valve has to be since it mixes hot and cold water together to get the proper temperature, (assuming the heater is actually running when you have the problem).

There will be a pair of wires connected to the thermostat contacts in the controller...see where they go. That is what turns the pump on and off - it likely goes to an aquastat. Depending on how the tankless is setup would determine what you might do. The tankless' input could be a trigger telling it the pump is on, or it could be an output to turn the pump on (i.e., it could be using the internal aquastat as the control input to the pump's controller. Often, control voltages are 24vac, but the pump needs 120vac to run, so you can't just hook it up directly. You'd have to read and understand the manual and diagrams to sort this out.

HJ - I don't see anything that obviously looks like a check valve or tempering valve at the heater but it's possible that there's one there and I'm just missing it? Here's a better picture... What do you think?

Hackney - Good question on the purpose of the relay. I'm not sure honestly. This is the way the builder installed the system. I'm sort of wondering whether the relay box is needed.

Jim - There's a skinny wire going from the heater to the relay. You can see it in the picture above. Then a set of larger diameter wires going from the relay to the pump. The wires going to the pump are probably power wires. The skinny wire going from the heater to the relay must be telling the relay when to send power to the pump. To your earlier point, it looks like the relay is really just a big switch that gives the pump power. It appears that the heater is actually the thing deciding when to turn the recirc pump on or off.

I know that when piping a recirculating line on a flash heater nowadays the heater requires a storage tank (with elements), roughly equal to the size of the recirculating line to keep the recirculating loop hot. It doesn't cycle through the flash heater but circulates through the system. Rinnai has a couple of piping diagrams of how to do it.

Yeah, I saw lots of information about tankless hot water heaters not being supported with recirc pumps for various reasons. The manual on this TK3 said it has a built in controller for recirc pumps and supports them. Of course that doesn't mean mine is set-up correctly for one.

I've been wondering if there's some sort of maintenance I should do. It sounds like I should look into this mineral cleaning process. I lived in West Texas for a while and that water was AWEFUL for leaving deposits. I live in Washington state now and the water seems to be way lower in mineral content but I'll look into the cleaning thing. Thanks for that suggestion.

I run a pretty hot shower and we used to not have the problem so I don't think cool shower is it.

Good point on the 3-zone controller being easier to get as a possible solution for why it's there vs. a simple timer.

Aaron

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You're right, unless you're on a particularly bad well, the water west of the Cascades has MUCH less mineral content than typical W-TX. It's likely this unit could run for decades without liming up.

It's possible, even likely, that the hit of cold water you're experiencing is the standard "cold water sandwich" aspect of tankless HW heaters, which is not cured by recirculation systems. With every draw there is a necessary ignition delay that lets cold water pass through the heat exchanger with out heating it.

With a recirculation system the hot water arrives at the tap and turns off the pump, so the water is always nice and hot when water is first drawn. But when the draw begin there are a few seconds of flow through the tankless prior to establishing the flame that will be unheated, but then followed by more hot water. To reduce or eliminate that feature a small amount of local hot water storage plumbed anywhere between the tankless and the tap works- even a 2-3 gallon point-of-use electric hot water heater (not wired up) provides sufficient dilution for that quart of cold or tepid water to bring it close to the average temp, smoothing the dip in temp experienced at the tap to near-undetectable levels.

I have bottled water quality water. Do you have a hot water-electric type teapot? Mine has the element under a stainless steel bottom plate, and after about 2 months, the bottom is fully covered with mineral deposits. Imagine what goes on inside a pipe for 3 years with a gas flame on it.

Pass thru water heaters are good for a little apartment or cabin with minimal use - unless you want to add into your life the pleasure of acid washing out a water heater every year or more. America had it made with tank water heaters that used to last 20+ years without much care. Autos have been gadgetized into repair oblivion, and now water heaters are being foisted upon us with hundreds of parts to go berzerk. I have a friend that took a sledge hammer to his flo thru after spending many thousands, and installed a 300$ electric job.

Any good life cost anaylsis shows flo thrus in big houses to be a huge joke on the consumer.

And the flo thru jokes requiring a tank water heater to make recirc work? That shows you how super salesmen screw you for their profit line. Why sell a tank rig with a 50$ profit, when the supplier can make 600$ on a POS flow-thru-plug-up-hot-cold-hot-cold mechanical nightmare?

" Hey honey - instead of dinner and movie this Saturday, lets acid wash our water heater! then we can check all our tire pressures, the well expansion tank pressure, and triple floss our teeth" .... "Good idea love, we can also trim the cat and dogs toenails"

I have bottled water quality water. Do you have a hot water-electric type teapot? Mine has the element under a stainless steel bottom plate, and after about 2 months, the bottom is fully covered with mineral deposits. Imagine what goes on inside a pipe for 3 years with a gas flame on it.

Pass thru water heaters are good for a little apartment or cabin with minimal use - unless you want to add into your life the pleasure of acid washing out a water heater every year or more. America had it made with tank water heaters that used to last 20+ years without much care. Autos have been gadgetized into repair oblivion, and now water heaters are being foisted upon us with hundreds of parts to go berzerk. I have a friend that took a sledge hammer to his flo thru after spending many thousands, and installed a 300$ electric job.

Any good life cost anaylsis shows flo thrus in big houses to be a huge joke on the consumer.

And the flo thru jokes requiring a tank water heater to make recirc work? That shows you how super salesmen screw you for their profit line. Why sell a tank rig with a 50$ profit, when the supplier can make 600$ on a POS flow-thru-plug-up-hot-cold-hot-cold mechanical nightmare?

" Hey honey - instead of dinner and movie this Saturday, lets acid wash our water heater! then we can check all our tire pressures, the well expansion tank pressure, and triple floss our teeth" .... "Good idea love, we can also trim the cat and dogs toenails"

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The tea kettle typically gets way hotter than a water heater. The hotter you heat water the more minerals precipitate out of it.......if it contains minerals.

While the amount would matter its the temp that makes the biggest difference,while your heating more the container is much larger with the water heater. and temp much higher with the kettle.. Thats why circ systems limit the water to 140......circulating it even makes it worse. How much does a circ system circulate....

Corrosive is a product of water chemistry not temperature. Precipitation of minerals is another.

At my elevation, I think boiling water is only about 200'F or less.

In any case, there is no argument that the flow thru beasts get limed up - you can see the rigs designed to acid wash them, and read the MFG's instructions. With an electric rig, you can go decades between a bottom of tank vacuum job.

Corrosive is a product of water chemistry not temperature. Precipitation of minerals is another.

At my elevation, I think boiling water is only about 200'F or less.

In any case, there is no argument that the flow thru beasts get limed up - you can see the rigs designed to acid wash them, and read the MFG's instructions. With an electric rig, you can go decades between a bottom of tank vacuum job.

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Water of the same chemistry wil be 4 times corrosive at 180 compared to 140. The hotter the water gets the less dense it becomes allowing the minerals to precipitate.

While the aggragate water temp doesn't get super hot, depending on the design, the flow, and how often it gets turned on, when the flow stops, there can be some very significant hot spots.

All tankless heaters will eventually require demineralization. It depends on your water quality (hardness), temperature setting, use pattern, and total usage when that needs to happen. It might be once a year, it might be longer, or shorter time period, but it will be necessary, or you'll notice a gradual drop off in performance (or maybe not until it gets really bad!). The minerals act like an insulator. The water has a relatively short path through the heater, and to raise the temp, there's a lot of heat being applied. The minerals will precipitate out of solution to a degree.

The hotter the water, the more reactive it is. That's one reason why there is a difference in the max flow rates for same sized pipes between hot and cold supplies - it's must remain slower for hot than cold at the max, or you'll eventually have problems.

Good debate on the degree to which temperature, flow, and mineral content all play a role in corrosion.

I've been staring at my plumbing this weekend trying to figure out how it's set-up.

If you look at the picture below you'll see that the cold water comes out of the wall and T's with one branch going to the inlet in the bottom of the water heater (bright flexible copper pipe) and the other branch going directly into the recirc pump.

The water comes out of the recirc pump and goes into the wall. It doesn't appear to ever go through the heater!? What am I missing.